I understand that chilodonella is a leaf shaped protozoan invisible to the naked eye manifested across the gill area, acompanied by excessive mucus production, labored breathing, clamped fins, and flashing. A light-blue opaque layer on the skin between the head and dorsal fin is characteristic of this illness. The skin begins to swell and fall off in strips in later stages.

I believe your description of your fish's problem does not fit this condition. Most likely you may have had an ammonia parameter spike while you were away, which can leave black smears or patches (burns) on any area of the body, head, or fins. Black is a sign of healing and is often found after an injury or burn, meaning that by now the problem is gone and your fish is simply recovering.

I would keep an eye on the water parameters and your fish and advise against taking action until further information is obtained. The black patch should go away within a week or so...like I said, just keep a close eye on it and keep tabs on the water quality.

If you can, maybe you could post pics?? Or describe it a bit closer? I'd also like to know how the fish in question is acting. Is she eating, swimming normally, all fins up?Always when you see something strange on your fish, do a water change instead, of 50-60% and see if that helps. Biggest reason why fish die is because their owners fret over nothing and start dumping medicines into the tank without knowing for sure what they're doing. (Been there, done that :'D)

If the behavior is all normal and she's eating well I wouldn't worry too much. Those are both signs that your goldfish is pretty healthy. I've noticed when I have an ammonia spike sometimes only one fish will get black marks and all of the others are uneffected.

As to why she was the only one affected, it could be simply that the fish is a bit weaker or more sensitive to water parameters than others.

It does look a lot like some kind of healing bruise, nothing worse So you just keep taking care of your fish! Like Goldiegirl says, as long as they eat and act perky, they're usually perfectly healthy

I didn't catch if you had a test kit or not, but if you're worried, you could take a sample of your water to a pet shop and ask them to test it for you! If the parameters are as follows: Ammonia 0, Nitrite 0, Nitrate 10-30 and pH around 6.5-8.5, all you need to do is a 50% water change